A so-called IC card in which a semiconductor integrated circuit device and an antenna are mounted exchanges information to/from a reader/writer device and a semiconductor integrated circuit device and realizes various functions such as transmission of data held in the IC card and retention of data transmitted from the reader/writer device.
According to the international standard ISO/IEC14443, an IC card having a contactless interface is called a PICC, and performs RF communication with a reader/writer device called a PCD. ISO stands for International Organization for Standardization, IEC stands for International Electrical Commission, PICC stands for Proximity Card, and PCD stands for Proximity Coupling Device.
For example, as described in the following non-patent literature 1, in type A of the international standard ISO/IEC14443, communication from a PCD to a PICC employs a coding method which is a modified miller coding in a modulation system with the degree of modulation of ASK 100%. However, in type B of the international standard ISO/IEC14443, communication from a PCD to a PICC employs the coding method which is an NRZ-L coding in a modulation system with a modulation ratio of ASK 10%. NRZ-L stands for Non Return to Zero-Level. ASK stands for Amplitude Shift Keying as one of digital modulation methods.
As described in the following non-patent literature 2, in initialization of the type B, a PICC of the type B waits for a request command of the type B in an idle state. The request command is for preparation to generate an application family identifier (AFI), an attribute information parameter (PARAM), and a cyclic redundancy check code (CRC) in a PICC of the type B. When the PICC of the type B detects a match of the AFI, a response to the request of the type B is transmitted to the PCD. The response includes a pseudo unique identifier (PUPI), application information (application data), protocol information, and a cyclic redundancy check code (CRC). After that, when the PICC of the type B receives a PICC selection command, it transmits a response to the command to the PCD and shifts to an active state. Further, transmission data of the type B is called a character as a frame, and the border of frames is determined by SOF (Start Of Frame) and EOF (End Of Frame). Each of the SOF and EOF includes one trailing edge and the logic “0” having a predetermined length.
In recent years, a communication technique called a near field communication technique (NFC) which simplifies and enlarges transactions in home appliances, digital media, wireless communication connection for consumers, contents, and business is being widely spread. The NFC technique is compatible with various existing communication systems and realizes a near field communication of the maximum baud rate of 847 Kbps in about 10 cm by using an radio frequency of 13.56 MHz. Particularly, the NFC technique is used for a cellular phone terminal having therein an IC card microcomputer (secure chip) having an electronic payment function. It aims to improve end-user convenience by utilizing the technique for various contactless electronic settlements such as payment for an item purchased in a shop by a contactless settlement and payment for transportation at a station. NFC stands for Near Field Communication.
The following non-patent literature 3 describes the NFC of the international standard ISO/IEC18092. In communication of a transfer rate of 212 Kbps to 424 Kbps of the NFC of the ISO/IEC 18092, the Manchester coding is used in ASK modulation whose modulation percentage is 8% to 30%. In a passive communication mode, an initiator generates an RF electric field for supplying energy to a target. On the other hand, in an active communication mode, the initiator and the target alternately generate the RF electric field.
In initialization of communication at a transfer rate of 106, 212, or 424 Kbps of ISO/IEC 18092, an application switches to the active communication mode and selects one of the three transfer rates. In initialization in a passive communication mode of the transfer rate of 212 or 424 Kbps, a preamble is inserted before a data packet. The preamble includes the minimum 48 bits in which the logic “0” is encoded.
As described above, in so-called downlink communication which is data communication from a reader/writer device conformed with the type B of the international standard ISO/IEC 14443 to an IC card, as described in the non-patent literatures 1 and 2, a so-called amplitude modulating method of changing the amplitude of a carrier signal is used.
The downlink communication conformed with the type B of the international standard ISO/IEC 14443 described in the non-patent literature 1 is information transmission according to the so-called amplitude modulation method of partly modulating the amplitude of a carrier signal which is a high-frequency AC signal by downlink communication data, and the downlink communication data is coded by NRZ-L.
The SOF (communication start signal) is added to the head of downlink communication data conformed with the type B of the international standard ISO/IEC 14443 described in the non-patent literature 1, and the EOF (communication end signal) is added to the end of the downlink communication data.
Further, the downlink communication conformed with the type B of the international standard ISO/IEC 14443 described in the non-patent literature 2 is information transmission according to the so-called amplitude modulation method of partly modulating the amplitude of a carrier signal which is a high-frequency AC signal by transmission data.
Downlink communication data conformed with the international standard ISO/IEC 18092 described in the non-patent literature 3 is coded by a Manchester code, and a preamble is added at the head of downlink communication data.
As described above, a semiconductor integrated circuit device mounted on an IC card receives a high frequency signal supplied from a reader/writer device by an antenna mounted on a contactless IC card, rectifies and smoothes voltage generated across both ends of the antenna, and generates an internal voltage necessary for the operation of an internal circuit.
The power received by the antenna mounted on the IC card changes according to the communication distance to the reader/writer device and, on the other hand, the signal amplitude modulated by the downlink communication data which is transmitted from the reader/writer device changes. Generally, when the communication distance is short, the modulation signal amplitude received by the IC card is large. With distance from the communication distance, the modulation signal amplitude decreases.
Therefore, to increase the communication distance of the IC card, a receiver circuit capable of performing demodulation even in the case where an amplitude change formed by downlink communication data is small has to be realized.
As a receiver circuit mounted on an IC card adapted to a communication interface using an amplitude modulation method as downlink communication conformed with the type B of the international standard ISO/IEC 14443 described in the non-patent literatures 1 and 2, a receiver circuit disclosed in the patent literature 1 can be used.
The receiver circuit disclosed in the patent literature 1 rectifies an AC signal at both ends of an antenna mounted on an IC card and eliminates an unnecessary frequency component by a filter circuit. An output signal of the filter circuit is supplied to an inverting input terminal (−) of an operational amplifier via a capacitor. By detecting a change point in an output signal of the filter circuit by the operational amplifier and a feedback path, a signal amplitude is amplified, and downlink communication data transmitted from a reader/writer device can be received.
A receiver circuit disclosed in the patent literature 2 rectifies an AC signal at both ends of an antenna mounted on an IC card and eliminates an unnecessary frequency component by a filter circuit. From a signal obtained by eliminating the unnecessary frequency component, two signals of a low-frequency signal and a high-frequency signal are generated. From the two signals, change points in positive and negative directions are detected by a comparator having a hysteresis characteristic. By binarizing the change points by a bistable circuit, data is demodulated.